Wireless Local Area Networks (WLANs) and Wireless Sensor Networks (WSNs) Primer - PowerPoint PPT Presentation

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Wireless Local Area Networks (WLANs) and Wireless Sensor Networks (WSNs) Primer

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... with interference from microwave ovens, cordless phones and garage door openers. ... Data sent from sensors to more powerful computers for storage and analysis. ... – PowerPoint PPT presentation

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Title: Wireless Local Area Networks (WLANs) and Wireless Sensor Networks (WSNs) Primer


1
Wireless Local Area Networks (WLANs) and
Wireless Sensor Networks (WSNs) Primer
2
Wireless Local Area Networks
  • The proliferation of laptop computers and other
    mobile devices (PDAs and cell phones) created an
    obvious application level demand for wireless
    local area networking.
  • Companies jumped in, quickly developing
    incompatible wireless products in the 1990s.
  • Industry decided to entrust standardization to
    IEEE committee that dealt with wired LANs
  • namely, the IEEE 802 committee!!

3
IEEE 802 Standards Working Groups
802.15.4 ZigBee
Figure 1-38. The important ones are marked with
. The ones marked with ? are hibernating. The
one marked with gave up.
Tanenbaum slide
4
Classification of Wireless Networks
  • Base Station all communication through an
    Access Point (AP) note hub topology. Other
    nodes can be fixed or mobile.
  • Infrastructure Wireless AP is connected to
    the wired Internet.
  • Ad Hoc Wireless wireless nodes communicate
    directly with one another.
  • MANETs (Mobile Ad Hoc Networks) ad hoc nodes
    are mobile.

5
Wireless LANs
  • Figure 1-36.(a) Wireless networking with a base
    station. (b) Ad hoc networking.

6
The 802.11 Protocol Stack
Figure 4-25. Part of the 802.11 protocol stack.
  • Note ordinary 802.11 products are no longer
    being manufactured.

Tanenbaum slide
7
Wireless Physical Layer
  • Physical layer conforms to OSI (five options)
  • 1997 802.11 infrared, FHSS, DSSS FHSS and DSSS
    run in the 2.4GHz band
  • 1999 802.11a OFDM and 802.11b HR-DSSS
  • 2001 802.11g OFDM
  • 802.11 Infrared
  • Two capacities 1 Mbps or 2 Mbps.
  • Range is 10 to 20 meters and cannot penetrate
    walls.
  • Does not work outdoors.
  • 802.11 FHSS (Frequence Hopping Spread Spectrum)
  • The main issue is multipath fading.
  • PD The idea behind spread spectrum is to
    spread the signal over a wider frequency to
    minimize the interference from other devices.
  • 79 non-overlapping channels, each 1 Mhz wide at
    low end of 2.4 GHz ISM band.
  • The same pseudo-random number generator used by
    all stations to start the hopping process.
  • Dwell time min. time on channel before hopping
    (400msec).

8
Wireless Physical Layer
  • 802.11 DSSS (Direct Sequence Spread Spectrum)
  • The main idea is to represent each bit in the
    frame by multiple bits in the transmitted signal
    (i.e., it sends the XOR of that bit and n random
    bits).
  • Spreads signal over entire spectrum using
    pseudo-random sequence (similar to CDMA see
    Tanenbaum sec. 2.6.2).
  • Each bit transmitted using an 11-bit chipping
    Barker sequence, PSK at 1Mbaud.
  • This yields a a capacity of 1 or 2 Mbps.

Figure 2.37 Example 4-bit chipping sequence
PD slide
9
Wireless Physical Layer
  • 802.11a OFDM (Orthogonal Frequency Divisional
    Multiplexing)
  • Compatible with European HiperLan2.
  • 54 Mbps in wider 5.5 GHz band ? transmission
    range is limited.
  • Uses 52 FDM channels (48 for data 4 for
    synchronization).
  • Encoding is complex ( PSM up to 18 Mbps and QAM
    above this capacity).
  • E.g., at 54 Mbps 216 data bits encoded into into
    288-bit symbols.
  • More difficulty penetrating walls.

10
Wireless Physical Layer
  • 802.11b HR-DSSS (High Rate Direct Sequence Spread
    Spectrum)
  • 11a and 11b shows a split in the standards
    committee.
  • 11b approved and hit the market before 11a.
  • Up to 11 Mbps in 2.4 GHz band using 11 million
    chips/sec.
  • Note in this bandwidth all these protocols have
    to deal with interference from microwave ovens,
    cordless phones and garage door openers.
  • Range is 7 times greater than 11a.
  • 11b and 11a are incompatible!!

11
Wireless Physical Layer
  • 802.11g OFDM(Orthogonal Frequency Division
    Multiplexing)
  • An attempt to combine the best of both 802.11a
    and 802.11b.
  • Supports bandwidths up to 54 Mbps.
  • Uses 2.4 GHz frequency for greater range.
  • Is backward compatible with 802.11b.

12
802.11 MAC Sublayer Protocol
  • In 802.11 wireless LANs, seizing the channel
    does not exist as in 802.3 wired Ethernet.
  • Two additional problems
  • Hidden Terminal Problem
  • Exposed Station Problem
  • To deal with these two problems 802.11 supports
    two modes of operation
  • DCF (Distributed Coordination Function)
  • PCF (Point Coordination Function).
  • All implementations must support DCF, but PCF is
    optional.

13
Figure 4-26.(a)The hidden terminal problem. (b)
The exposed station problem.
Tanenbaum slide
14
The Hidden Terminal Problem
  • Wireless stations have transmission ranges and
    not all stations are within radio range of each
    other.
  • Simple CSMA will not work!
  • C transmits to B.
  • If A senses the channel, it will not hear Cs
    transmission and falsely conclude that A can
    begin a transmission to B.

15
The Exposed Station Problem
  • This is the inverse problem.
  • B wants to send to C and listens to the channel.
  • When B hears As transmission, B falsely assumes
    that it cannot send to C.

16
Distribute Coordination Function (DCF)
  • Uses CSMA/CA (CSMA with Collision Avoidance).
  • Uses one of two modes of operation
  • virtual carrier sensing
  • physical carrier sensing
  • The two methods are supported
  • 1. MACAW (Multiple Access with Collision
    Avoidance for Wireless) with virtual carrier
    sensing.
  • 2. 1-persistent physical carrier sensing.

17
Wireless LAN ProtocolsTan pp.269-270
  • MACA protocol solved hidden and exposed terminal
    problems
  • Sender broadcasts a Request-to-Send (RTS) and the
    intended receiver sends a Clear-to-Send (CTS).
  • Upon receipt of a CTS, the sender begins
    transmission of the frame.
  • RTS, CTS helps determine who else is in range or
    busy (Collision Avoidance).
  • Can a collision still occur?

18
Wireless LAN Protocols
  • MACAW added ACKs, Carrier Sense, and BEB done per
    stream and not per station.
  • Figure 4-12. (a) A sending an RTS to B.
  • (b) B responding with a CTS to A.

Tanenbaum slide
19
Virtual Channel Sensing in CSMA/CA
  • Figure 4-27. The use of virtual channel sensing
    using CSMA/CA.
  • C (in range of A) receives the RTS and based on
    information in RTS creates a virtual channel busy
    NAV(Network Allocation Vector).
  • D (in range of B) receives the CTS and creates a
    shorter NAV.

Tanenbaum slide
20
Virtual Channel Sensing in CSMA/CA
  • What is the advantage of RTS/CTS?
  • RTS is 20 bytes, and CTS is 14 bytes.
  • MPDU can be 2300 bytes.
  • virtual implies source station sets the
    duration field in data frame or in RTS and CTS
    frames.
  • Stations then adjust their NAV accordingly!

21
Figure 4-28.Fragmentation in 802.11
  • High wireless error rates ? long packets have
    less probability of being successfully
    transmitted.
  • Solution MAC layer fragmentation with
    stop-and-wait protocol on the fragments.

Tanenbaum slide
22
1-Persistent Physical Carrier Sensing
  • The station senses the channel when it wants to
    send.
  • If idle, the station transmits.
  • A station does not sense the channel while
    transmitting.
  • If the channel is busy, the station defers until
    idle and then transmits (1-persistent).
  • Upon collision, wait a random time using binary
    exponential backoff (BEB).

23
Point Coordinated Function (PCF)
  • PCF uses a base station to poll other stations to
    see if they have frames to send.
  • No collisions occur.
  • Base station sends beacon frame periodically.
  • Base station can tell another station to sleep to
    save on batteries and base stations holds frames
    for sleeping station.

24
DCF and PCF Co-Existence
  • Distributed and centralized control can co-exist
    using InterFrame Spacing.
  • SIFS (Short IFS) is the time waited between
    packets in an ongoing dialog (RTS,CTS,data, ACK,
    next frame)
  • PIFS (PCF IFS) when no SIFS response, base
    station can issue beacon or poll.
  • DIFS (DCF IFS) when no PIFS, any station can
    attempt to acquire the channel.
  • EIFS (Extended IFS) lowest priority interval
    used to report bad or unknown frame.

25
Figure 4-29. Interframe Spacing in 802.11.
Tanenbaum slide
26
A Few Wireless Details
  • 802.11b and 802.11g use dynamic rate adaptation
    based on ?? (algorithms internal to wireless card
    at the AP)
  • e.g. for 802.11b choices are 11, 5.5, 2 and 1
    Mbps
  • RTS/CTS may be turned off by default Research
    has shown that RTS/CTS degrades performance when
    hidden terminal is not an issue.
  • All APs (or base stations) will periodically send
    a beacon frame (10 to 100 times a second).
  • Beacon frames are also used by DCF to synchronize
    and handle nodes that want to sleep. The AP will
    buffer frames intended for a sleeping wireless
    client.
  • AP downstream/upstream traffic performance is
    asymmetric.
  • Wireless communication quality between two nodes
    can be asymmetric due to multipath fading.

27
Wireless Sensor Networks
  • Sensors small devices with low-power
    transmissions and energy limitations (e.g.,
    battery lifetime is often a BIG concern.)
  • The main distinction from traditional wireless
    networks is that the data traffic originates at
    the sensor node and is sent upstream towards the
    access point (AP) or base station that collects
    the data.
  • While the nature of data collection at the sensor
    is likely to be event driven, for robustness, the
    generation of sensor packets should be periodic
    if possible.

28
Tiered Architecture
  • Smaller sensors on the leaves of the tree
  • 1. Motes, TinyOS
  • 2. Strong ARM PDA running Linux
  • Battery powered, lifetime is critical.
  • Need to be able to adjust transmission power and
    permit sensor to go to sleep.
  • Second Tier
  • AP, base station or video aggregator
  • Data sent from sensors to more powerful computers
    for storage and analysis.

29
The Berkeley System
AP
AP
AP
sensor
sensor
Multiple hop tree topology
sensor
sensor
sensor
sensor
sensor
sensor
sensor
sensor
30
The Berkeley System
AP
AP
AP
AP range
sensor
sensor
sensor
sensor
sensor
sensor
Sensor range
sensor
sensor
sensor
sensor
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